1. Field of the Invention
This invention relates to processes for preparing iodoaromatic compounds. This invention further relates to the use of iodoaromatic compounds in the formation of triarylamine hole transport small molecules, which may be used in electrophotography.
2. Description of Related Art
Electrophotographic imaging members (i.e. photoreceptors) are well known. Electrophotographic imaging members are commonly used in electrophotographic (xerographic) processes and may comprise a photoconductive layer including a single layer or composite layers. These electrophotographic imaging members take many different forms. For example, layered photoresponsive imaging members, such as those described in U.S. Pat. No. 4,265,990 to Stolka et al., which is incorporated by reference in its entirety, are known in the art.
More advanced photoconductive photoreceptors containing highly specialized component layers are also known. For example, a multilayered photoreceptor employed in electrophotographic imaging systems sometimes includes one or more of a substrate, an undercoating layer, an intermediate layer, an optional hole or charge blocking layer, a charge generating layer over an undercoating layer and/or a blocking layer, and a charge transport layer (including a charge transport material in a binder). Additional layers such as one or more overcoating layer or layers are also sometimes included.
Various compounds are known for their uses as charge transporting materials, including hole transporting materials, in charge transporting layers of electrophotographic apparatuses, including pyrazoline compounds such as those disclosed in JP-B-37-10696, triarylamine compounds such as those disclosed in U.S. Pat. No. 3,180,730 to Klupfel et al., stilbene compounds as disclosed in published unexamined Japanese Patent Application JP-A-58-198043, hydrazone compounds as disclosed in JP-B-55-42380, oxadiazone compounds as disclosed in JP-B-34-5466, butadiene compounds as disclosed in published unexamined Japanese Patent Application JP-A-63-314554, and so on, are known. Among these compounds, triarylamine compounds are of particular importance, in view of high charge-transporting and hole-transporting ability (mobility), and various triarylamine compounds have been disclosed, e.g., in JP-A-1-280763, JP-A-2-178666, JP-A-2-178667, JP-A-2-178668, JP-A-2-178669, JP-A-2-178670, JP-A-2-190862, JP-A-2-190863, JP-A-2-230255, JP-A-3-78755, JP-A-3-78756, JP-A-3-78757, JP-A-3-114058, JP-A-4-133064, JP-A-4-193852, JP-A-4-312558, JP-A-5-19509, JP-A-5-80550, and JP-A-5-313386.
It is generally known that triarylamine compounds can be synthesized by coupling an arylamine compound with an aryl halide, usually an aryl bromide or an aryl iodide, using a copper catalyst. Aryl iodides are preferred due to the low reactivity of aryl bromides during tertiary amine formation.
Typically, large-scale production of triarylamine small molecules is accomplished by use of an Ullmann condensation reaction, such as those illustrated in reaction schemes (1) and (2):
[Insert Chemical Drawing (1)]

[Insert Chemical Drawing (2)]

The use of Ullmann condensation reactions in the production of tertiary amines, specifically triarylamines, is described in detail in, for example, U.S. Pat. No. 4,764,625 to Turner, et al., the entire disclosure of which is incorporated herein by reference.
In such Ullmann condensation reactions, aromatic halides, such as aryl iodides, are reacted with aromatic amine compounds in the presence of a base, a copper catalyst and, optionally, an inert solvent. Aromatic iodides possess a much higher reactivity than other aromatic halides in these reactions. Typically, aryl iodides have higher kinetic rates of product formation than other aromatic halides, as illustrated by the reduced reaction times necessary to produce higher yields of highly pure products than with other aromatic halides. Thus, aryl iodides are key substrates in the Ullmann condensation reactions traditionally used to manufacture triarylamine hole transport small molecules.
However, the benefits of using aryl iodides to form triarylamines by Ullmann condensation are not without cost. Aryl iodides are generally more expensive than aryl bromides or aryl chlorides.
The formation of aryliodides by iodinating an aromatic compound with a sulfuric acid catalyst in a mixed water/acetic acid solvent, using iodic acid and iodine, as shown in Ann., 634, 84 (1960) is known, as exemplified by the iodination of biphenyl in reaction scheme (3). Reaction scheme (3) represents a common, currently employed manufacturing method for 4-iodobiphenyl.
[Insert Chemical Drawing (3)]

In this reaction, biphenyl is iodinated by a hypervalent iodine species, prepared from elemental iodine and periodic acid in a strongly acidic solution. Preparation of monoiodo compound by this method is difficult, at least because over-iodination occurs. In the reaction scheme (3) above, the reaction mixture contains starting material, 4-iodobiphenyl, and 1,4-diiodobiphenyl. In order to prepare the mono-iodo compound for use in an Ullmann condensation, extensive purification by recrystallization is necessary, and yields of only about 60% can be obtained.
Iodination with periodic acid is carried out with ease. However the reaction selectivity is low. The reaction product is a mixture of monoiodo and diiodo compounds. When a subsequent amination reaction is carried out using this mixture, the reaction product after amination also comprises a mixture. Since impurities in the amination product have detrimental effects on the electrical characteristics of the charge transport material, purification of the product is required. The molecular weight of the impurities is large enough to render purification by distillation, etc., impractical. Thus, a very expensive purification method, such as a column purification, etc., must be used. In addition to the expense of iodine, a method in which many diiodo byproducts are formed is a costly process for preparing arylamine compounds.
Also, the solubility of diiodo compounds is very low, the diiodo compound cannot be removed from the product by recrystallization, which may be easily incorporated into industrial operations at low cost. Therefore, for product mixtures containing about 10% or more of diiodo compounds, purification by distillation is required.
The monoiodo compound, in contrast, has a high boiling point and a high melting point. For these reasons, high vacuum conditions are required in the distillation of monoiodo compound, and the product recovered by distillation is apt to solidify and become difficult to handle. When the mixture to be subjected to distillation contains a large amount of diiodo compounds, distillation must be carried out multiple times. The purity of the product subjected to a single distillation is lowered by “splashing” of the distillation mixture, resulting in contamination of the distillation product by diiodo compounds. Thus, fractional distillation is required to purify the monoiodo product from a reaction mixture containing a large amount of diiodo compounds. The more involved purification process complicates operation and increases manufacturing costs.
Also, Ann., 634, 84 (1960) describes carrying out the iodination reaction in a saturated solution of an aromatic compound to increase the selectivity of monoiodo compound formation. This method, however, does not provide a low-cost product having sufficient purity as a raw material used for a charge transporting material.
The present invention is provided to solve the problems described above. That is, the present invention provides a low cost route to iodoaromatic molecules, having high yields of highly pure monoiodo compounds, and thus a low cost route to triarylamine hole transport small molecules.